In this linear instability study, we examine the incipience of depth averaged circulations, which are related to rip currents in the surf zone, due to interactions of waves and currents on alongshore uniform beaches. We consider normally incident waves, subject to infinitesimal perturbations which are alongshore periodic. Circulations are described by the depth-time averaged shallow water equations which include wave forcing and bottom friction. The slow modulation of the wave field is described by the ray theory. We assume linear shallow water waves and a saturated surf zone. The properties of the instability are then examined using two parameters, representing the effects of the incident wave height and bottom friction dissipation.